1. Technical Field of the Invention
The present invention relates generally to a fuel injection control apparatus for an internal combustion engine which may be employed in a common rail fuel injection system designed to learn the quantity of fuel injected into the engine to correct or compensate for a difference between a target quantity of fuel to be sprayed from a fuel injector and a quantity of fuel actually sprayed from the fuel injector.
2. Background Art
Fuel injection systems are known which are designed to learn a variation in quantity of fuel sprayed from fuel injectors due to the aging thereof and correct a control signal to be outputted to the fuel injector so as to compensate for such a variation. For example, Japanese Patent First Publication Nos. 2005-36788 (U.S. Pat. No. 6,907,861 B2 assigned to the same assignee as that of this application) and 2007-138750 (US 2007/0112502 A1) teach such compensating techniques. Particularly, fuel injection systems for diesel engines designed to perform the pilot injection before the main injection in order to reduce NOx emissions and burning noises are required to learn the quantity of fuel actually injected into the engine to ensure the accuracy in spraying a small quantity of fuel through fuel injectors.
The fuel injection system, as taught in Japanese Patent First Publication No. 2005-36788 (U.S. Pat. No. 6,907,861 B2), works to output a control signal to a fuel injector to spray a learning quantity of fuel into the engine and monitor a resulting change in speed of the engine to calculate the quantity of fuel actually sprayed by the fuel injector for correcting the control signal so as to compensate for a difference between the learning quantity and the actual quantity.
The fuel injection system, as taught in Japanese Patent First Publication No. 2007-138750 (US 2007/0112502 A1), is designed to output a control signal to a fuel injector to spray a learning quantity of fuel into an internal combustion engine, calculate a rate of slippage in rotation between a driving and a driven member of a power train through which output torque of the engine is transmitted, and determine the quantity of fuel actually sprayed from the fuel injector based on a rise in speed of the engine and the rate of slippage to correct the control signal so as to compensate for a difference between the learning quantity and the actual quantity.
However, direct addition of variation in physical load to the driving member of the power train or indirect thereof to the driven member will cause an increase in speed of the engine arising from the injection of the learning quantity of fuel thereinto to change as compared to when the variation in load is not exerted on the driving member or the driven member, thus resulting in an error in calculating the actual quantity of fuel injected into the engine based on the change in speed of the engine. This leads to an error in determining the correction factor based on the difference between the learning quantity and the actual quantity of fuel injected into the engine.